Pressure transducer with composite diaphragm

ABSTRACT

A method of forming a composite diaphragm for a pressure transducer is disclosed. The method comprises providing a substrate layer having a first conductivity type and a first surface. Positive implants are deposited in the first surface of the substrate layer, and an epitaxial layer is grown on the first surface of the substrate layer so that the positive implants form positive diffusions in the epitaxial layer. An oxide pattern is formed on the epitaxial layer, and a top layer is deposited over the epitaxial layer and oxide pattern. The substrate layer and positive diffusions of the epitaxial layer are then etched to form the composite diaphragm. The positive diffusions can be patterned so that the resulting etched structure has improved diaphragm performance characteristics. For example, the remaining pattern can include a plurality of bosses and interconnecting battens so that the diaphragm has a relatively high burst pressure and a high output signal with improved linearity at low pressures.

FIELD OF THE INVENTION

[0001] The present invention generally relates to pressure transducersand, more particularly, to the diaphragm portions of such transducers.

BACKGROUND OF THE INVENTION

[0002] Pressure transducers that use piezoresistors are well known inthe art. Generally, such transducers are formed with a silicon substrateand an epitaxial layer, which is grown on the substrate. A portion ofthe substrate is removed, leaving a thin, flexible diaphragm portion.The piezoresistors are located in the diaphragm portion to form apressure transducer.

[0003] In operation, at least one surface of the diaphragm is exposed toa process pressure. The diaphragm deflects according to the magnitude ofthe pressure, and this deflection bends the attached piezoresistors.Bending of the diaphragm creates changes in the resistance value of thepiezoresistors, which is reflected as a change in the output voltagesignal of a resistive bridge formed at least partially by thepiezoresistors.

[0004] The substrate and epitaxial layers are commonly formed of singlecrystal silicon. Diaphragm portions formed of single crystal siliconproduce adequate results for pressures ranging from five inches H₂O to6,000 PSI. Such material does not, however, produce a high output signalwith sufficient linearity at pressures below 5 inches H₂O.

SUMMARY OF THE INVENTION

[0005] The following summary of the invention is provided to facilitatean understanding of some of the innovative features unique to thepresent invention and is not intended to be a full description. A fullappreciation of the various aspects of the invention can be gained bytaking the entire specification, claims, drawings, and abstract as awhole.

[0006] In accordance with certain aspects of the present invention, amethod of forming a composite diaphragm for a pressure transducer orsimilar device is provided. The method comprises providing a substratelayer having a first conductivity type, the substrate layer having afirst surface. Positive implants are deposited in the first surface ofthe substrate layer, and an epitaxial layer is grown on the firstsurface of the substrate layer so that the positive implants formpositive diffusions in the epitaxial layer. An oxide pattern is formedon the epitaxial layer, and a top layer is deposited over the epitaxiallayer and oxide pattern. The substrate layer and positive diffusions ofthe epitaxial layer are etched to form the composite diaphragm.

[0007] In accordance with additional aspects of the present invention, acomposite diaphragm is provided for use in a pressure sensor or likedevice. The diaphragm comprises a first layer of silicon nitride and asecond layer attached to the silicon nitride layer and comprising apressure sensor pattern of silicon material.

[0008] The novel features of the present invention will become apparentto those of skill in the art upon examination of the following detaileddescription of the invention or can be learned by practice of thepresent invention. It should be understood, however, that the detaileddescription of the invention and the specific examples presented, whileindicating certain embodiments of the present invention, are providedfor illustration purposes only because various changes and modificationswithin the scope of the invention will become apparent to those of skillin the art from the detailed description of the invention and claimsthat follow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The accompanying figures, in which like reference numerals referto identical or functionally-similar elements throughout the separateviews and which are incorporated in and form part of the specification,further illustrate the present invention and, together with the detaileddescription of the invention, serve to explain the principles of thepresent invention.

[0010]FIG. 1 is a bottom view of a diaphragm portion of the pressuretransducer illustrating a preferred boss and batten structure.

[0011] FIGS. 2A-2G illustrate various steps during the fabrication ofthe pressure transducer.

[0012] It should be understood that the drawings are not necessarily toscale and that the embodiments are illustrated using graphic symbols,phantom lines, diagrammatic representations and fragmentary views. Incertain instances, details which are not necessary for an understandingof the present invention or which render other details difficult toperceive may have been omitted. It should be understood, of course, thatthe invention is not necessarily limited to the particular embodimentsillustrated herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0013] Referring initially to FIG. 1, a bottom view of a diaphragm 10for a pressure transducer is shown. The diaphragm 10 includes a layer 12of first diaphragm material formed in a solid square having a length “L”on each side. A second, patterned layer 14 of diaphragm material isattached to the first layer 12. As illustrated in FIG. 1, the secondlayer 14 comprises four bosses 16 interconnected by four battens 18.While the boss and batten structure shown in FIG. 1 is preferred, itwill be appreciated that a variety of different patterns can be formedin accordance with the present invention. Piezoresistors 19 are attachedto the second layer 14 at the base of each boss 16.

[0014] To form the pressure transducer diaphragm 10 shown in FIG. 1, thesteps depicted in FIGS. 2A-2G are followed. FIG. 2A shows the formationof a substrate 20 having a top surface 22 and a bottom surface 24. Thesubstrate 20 can be formed in a variety of known manners and cancomprise any one of a variety of known substrate materials. In thepreferred embodiment, the substrate comprises single crystal silicondoped with a p-type material.

[0015] In FIG. 2B, the top surface 22 of the substrate 20 is coated witha photo resist pattern 26. A p-type material is then directed toward thetop surface 22, such as with an ion implanter, so that the exposed areasof the top surface 22 receive the p-type material. The photo resistlayer 26 is removed and an n-type epitaxial layer 28 is grown on the topsurface 22, as best shown in FIG. 2C. The areas of the top surface 22that were exposed during the p-type implant create p-type diffusions 30in the n-type epitaxial layer 28.

[0016] As shown in FIG. 2D, an oxide layer 32 is deposited over theepitaxial layer 28, and a second photo resist layer 34 is patterned onthe oxide layer 32. The photo resist layer 34 and exposed areas of theoxide layer 32 are bombarded with a p-type material to create p-typediffusions 36 in the epitaxial layer 28, as best shown in FIG. 2E. A toplayer of diaphragm material 38 is deposited on top of the oxide layer32, and a bottom layer of diaphragm material 40 is deposited over thebottom layer 24 of the substrate 20 (FIG. 2F). In the preferredembodiment, the top layer 38 and bottom layer 40 comprise siliconnitride. The top and bottom layers 38, 40 can be deposited in a knownmanner, such as by chemical vapor deposition, plasma deposition, or RFsputtering.

[0017] The composite structure is then etched so that the p-typediffusions 30, 36 and much of the substrate 20 are removed. During theetching step, the composite structure is preferably placed in a tank ofetchant and a stop-etch process is used to remove the p-type material. Astop-etch process is described in commonly assigned U.S. Pat. No.5,360,521, which is incorporated herein by reference.

[0018] After the etching step, the structure shown in FIG. 2G remains.As shown therein, the top layer 38 has a substantially uniform thicknessacross the entire diaphragm, with remaining portions of the n-typeepitaxial layer 28 attached to the top layer 38. In the illustratedembodiment, the top layer 38 corresponds to the first layer of diaphragmmaterial 12 shown in FIG. 1. Similarly, the epitaxial layer 28corresponds to the patterned layer 14, where the remaining portions ofthe layer 28 form the bosses 16 and battens 18. It will be appreciated,however, that any desirable pattern can be formed in addition to thepreferred boss and batten pattern illustrated at FIG. 1. The remainingsubstrate and bottom layers 20, 40 provide structure for mounting thediaphragm 10 as necessary.

[0019] The resulting transducer structure provides a diaphragm having acomposite construction that can be adapted to particular designspecifications. The boss and batten pattern illustrated at FIG. 1produces a high output signal with good linearity at low pressurelevels, such as 5 inches H₂O. Each boss 16 is formed with an outsidewidth “A” and a length “B.” The battens 18 intersect each associatedboss 16 at an edge distance “C.” Each batten 18 comprises a pair ofangled leg portions 46 having a width “D” and a pair of connectingportions 50 having a length “E.” Each connecting portion 50 has a width“F” at the point of intersection with an associated boss 16. The legportions 46 intersect at a point 56 that is an equal distance “G” fromcenter lines 60, 62 of the bosses 16. Each connecting portion 50intersects in an associated 16 at an angle “β”. Furthermore, each boss16 is preferably tapered at an angle “θ”.

[0020] In a representative boss and batten embodiment, A is 200 μm, B is560 μm, C is 180 μm, D is 115 μm, E is 225 μm, F is 15 μm, G is 180 μm,L is 1400 μm, β is 60 degrees, and θ is 6 degrees. The top layer 38 ispreferably approximately 2.0 μm and the intermediate layer 30 isapproximately 10.5 μm, for an overall diaphragm thickness ofapproximately 12.5 μm. Each of these dimensions can be adjusted to varythe performance characteristics of the diaphragm 10. For example, thespan, defined herein as the algebraic difference between limits of thepressure range, can be increased by decreasing the boss outside width“A,” the leg portion width “D,” the connecting portion with “F,” theintersection point distance “G,” and the intersect angles “β,” or byincreasing the edge distance “C,” the connecting portion length “E,” andthe boss taper angle “θ.” Furthermore, a terminal base linearity of thediaphragm 10 can be decreased by decreasing the boss outside width “A,”the boss length “B.” the connecting portion length “E,” and the bosstaper angle “θ,” or by increasing the edge distance “C,” leg portionwidth “D,” connecting portion width “F,” intersection point distance“G,” and intersect angle “β.” Furthermore, it will be appreciated thatthe span and terminal base linearity will increase as the length “L” ofthe diaphragm increases.

[0021] The embodiments and examples set forth herein are presented tobest explain the present invention and its practical application and tothereby enable those skilled in the art to make and utilize theinvention. Those skilled in the art, however, will recognize that theforegoing description and examples have been presented for the purposeof illustration and example only. Other variations and modifications ofthe present invention will be apparent to those of skill in the art, andit is the intent of the appended claims that such variations andmodifications be covered. The description as set forth is not intendedto be exhaustive nor to limit the scope of the invention. Manymodifications and variations are possible in light of the above teachingwithout departing from the spirit and scope of the following claims. Itis contemplated that the use of the present invention can involvecomponents having different characteristics. It is intended that thescope of the present invention be defined by the claims appeaded hereto,giving full cognizance to equivalents in all respects.

[0022] The embodiments of an invention in which an exclusive property orright is claimed are defined as follows:

1. A method of forming a composite diaphragm for a pressure device, the method comprising the steps of: providing a substrate layer having a first conductivity type, the substrate layer having a first surface; depositing implants in the first surface of the substrate layer; growing an epitaxial layer on the first surface of the substrate layer; the implants forming diffusions in the epitaxial layer; forming an oxide pattern on the epitaxial layer and oxide pattern; and depositing a top layer over the epitaxial layer and oxide pattern; and etching the substrate layer and diffusions of the epitaxial layer to form the composite diaphragm.
 2. The method of claim 1, wherein the implants are positive implants and the diffusions are positive diffusions.
 3. The method of claim 1, in which the substrate layer comprises a p-type single crystal silicon material.
 4. The method of claim 1, in which the positive implants are deposited using an ion implanter.
 5. The method of claim 1, in which the top layer comprises silicon nitride.
 6. The method of claim 5, in which the substrate layer comprises a p-type single crystal silicon material.
 7. The method of claim 1, in which the etching step comprises a stop-etch process.
 8. The method of claim 2, in which portions of the epitaxial layer not formed with the positive diffusions form a pattern.
 9. The method of claim 8, in which the pattern comprises a plurality of bosses.
 10. The method of claim 9, in which the pattern further comprises a plurality of battens extending between adjacent bosses.
 11. The method of claim 10, in which the substrate layer comprises a p-type single crystal silicon material.
 12. The method of claim 10, in which the top layer comprises silicon nitride.
 13. The method of claim 12, in which the substrate layer comprises a p-type single crystal silicon material.
 14. A composite diaphragm for use in a pressure device, the diaphragm comprising: a first layer of silicon nitride; and a second layer attached to the silicon nitride layer and comprising a pressure sensor pattern of silicon material.
 15. The diaphragm of claim 14, further comprising a third layer intermediate the first and second layers, wherein the third layer comprises an epitaxial layer of single crystal silicon.
 16. The diaphragm of claim 15, further comprising a patterned oxide layer deposited between the second and third layers.
 17. The diaphragm of claim 14, in which the second layer comprises a p-type single crystal silicon.
 18. The diaphragm of claim 14, in which the pattern comprises a plurality of bosses.
 19. The diaphragm of claim 18, in which the pattern further comprises a plurality of battens extending between adjacent bosses.
 20. The diaphragm of claim 19, in which each batten includes a pair of intersecting leg portions and a pair of connecting portions adapted to engage adjacent bosses.
 21. The diaphragm of claim 14, wherein the second layer comprises a pressure sensor pattern of single crystal silicon.
 22. A silicon based diaphragm for a piezoresistor capable of producing a usable, substantially linear output signal at less than 5 inches of H₂O of input pressure.
 23. The silicon based diaphragm of claim 22 being capable of producing the usable, substantially linear output signal substantially through a range of 0 to 5 inches of H₂O of input pressure.
 24. The silicon based diaphragm of claim 22 comprising a silicon nitride layer abutting a single crystal silicon layer.
 25. The silicon based diaphragm of claim 22 being capable of producing the usable, substantially linear output signal substantially through a range of 0 to 5 inches of H₂O of input pressure. 